Problem: Luis is 3 times as old as Kevin and is also 4 years older than Kevin. How old is Luis?
Answer: We can use the given information to write down two equations that describe the ages of Luis and Kevin. Let Luis's current age be $l$ and Kevin's current age be $k$ $l = 3k$ $l = k + 4$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $l$ is to solve the second equation for $k$ and substitute that value into the first equation. Solving our second equation for $k$ , we get: $k = l - 4$ . Substituting this into our first equation, we get the equation: $l = 3$ $(l - 4)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $l = 3l - 12$ Solving for $l$ , we get: $2 l = 12$ $l = 6$.